Optimizing tag forwarding in a two level cache system from level one to lever two controllers for cache coherence protocol for direct memory access transfers

ABSTRACT

A second level memory controller uses shadow tags  711  to implement snoop read and write coherence. These shadow tags are generally used only for snoops intending to keep L2 SRAM coherent with the level one data cache. Thus updates for all external cache lines are ignored. The shadow tags are updated on all level one cache allocates and all dirty and invalidate modifications to data stored in L2 SRAM. These interactions happen on different interfaces, but the traffic on that interface includes level one data cache accesses to both external and level two directly addressable lines. These interactions create extra traffic on these interfaces and creating extra stalls to the CPU. Thus in this invention shadow tags are updated only on a subset of less than all updates of the level one tags.

CLAIM OF PRIORITY

This application claims priority under 35 U.S.C. 119(e)(1) to U.S.Provisional Application No. 61/387,283 filed Sep. 28, 2010.

TECHNICAL FIELD OF THE INVENTION

The technical field of this invention is cache in digital dataprocessors.

BACKGROUND OF THE INVENTION

A data processor to which the present invention is applicable employs atwo-level memory subsystem. The level one memory includes instructioncache (L1I) and data cache (L1D) and the level two memory containsdirectly addressable memory (SRAM), level two cache or both. The SRAM atlevel two can be cached within level one. Direct memory access (DMA)units can directly access the SRAM at level two. Keeping centralprocessing unit (CPU) and direct memory access (DMA) data transfers tothe level two memory coherent is important to the programmability of thedevice. Making this efficient is important to the performance of thedevice.

SUMMARY OF THE INVENTION

The data processor to which the present invention is applicable includesa copy of the level one data cache tag memory in the level twocontroller. The level two controller uses this shadow copy of the tagsto implement its snoop read and write coherence. The level twocontroller tracks the status of level one data cache lines. Since thisshadow copy of the tags is used only for snoops intending to keep leveltwo directly addressable memory coherent with the level one data cache,the information for all external cache lines are ignored. The shadowcopy of the level one data cache tag RAM in the level two controller isupdated on all level one data cache allocates and all dirty andinvalidate modifications. These interactions happen on differentinterfaces, but the traffic on that interface includes level one datacache accesses to both external and level two directly addressablelines. That creates extra traffic on these interfaces and also creatingextra stalls to the CPU. Thus the shadow tags are updated only on asubset of less than all updates of the level one tags.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of this invention are illustrated in thedrawings, in which:

FIG. 1 illustrates the organization of a typical digital signalprocessor to which this invention is applicable (prior art);

FIG. 2 illustrates details of a very long instruction word digitalsignal processor core suitable for use in Figure (prior art);

FIG. 3 illustrates the pipeline stages of the very long instruction worddigital signal processor core illustrated in FIG. 2 (prior art);

FIG. 4 illustrates the instruction syntax of the very long instructionword digital signal processor core illustrated in FIG. 2 (prior art);

FIG. 5 illustrates the details of a set of typical prior art cache lines(prior art);

FIG. 6 illustrates a computing system including a local memory arbiteraccording to an embodiment of the invention;

FIG. 7 is a further view of the digital signal processor system of thisinvention illustrating the various cache controllers; and

FIGS. 8A and 8B together form a flow chart illustrating when shadow tagsthe level two cache controller update.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates the organization of a typical digital signalprocessor system 100 to which this invention is applicable (prior art).Digital signal processor system 100 includes central processing unitcore 110. Central processing unit core 110 includes the data processingportion of digital signal processor system 100. Central processing unitcore 110 could be constructed as known in the art and would typicallyincludes a register file, an integer arithmetic logic unit, an integermultiplier and program flow control units. An example of an appropriatecentral processing unit core is described below in conjunction withFIGS. 2 to 4.

Digital signal processor system 100 includes a number of cache memories.FIG. 1 illustrates a pair of first level caches. Level one instructioncache (L1I) 121 stores instructions used by central processing unit core110. Central processing unit core 110 first attempts to access anyinstruction from level one instruction cache 121. Level one data cache(L1D) 123 stores data used by central processing unit core 110. Centralprocessing unit core 110 first attempts to access any required data fromlevel one data cache 123. The two level one caches are backed by a leveltwo unified cache (L2) 130. In the event of a cache miss to level oneinstruction cache 121 or to level one data cache 123, the requestedinstruction or data is sought from level two unified cache 130. If therequested instruction or data is stored in level two unified cache 130,then it is supplied to the requesting level one cache for supply tocentral processing unit core 110. As is known in the art, the requestedinstruction or data may be simultaneously supplied to both therequesting cache and central processing unit core 110 to speed use.

Level two unified cache 130 is further coupled to higher level memorysystems. Digital signal processor system 100 may be a part of amultiprocessor system. The other processors of the multiprocessor systemare coupled to level two unified cache 130 via a transfer request bus141 and a data transfer bus 143. A direct memory access unit 150provides the connection of digital signal processor system 100 toexternal memory 161 and external peripherals 169.

FIG. 1 illustrates several data/instruction movements within the digitalsignal processor system 100. These include: (1) instructions move fromL2 cache 130 to L1I cache 121 to fill in response to a L1I cache miss;(2) data moves from L2 cache 130 to L1D cache 123 to fill in response toa L1D cache miss; (3) data moves from L1D cache 123 to L2 cache 130 inresponse to a write miss in L1D cache 123, in response to a L1D cache123 victim eviction and in response to a snoop from L2 cache 130; (4)data moves from external memory 161 to L2 cache 130 to fill in responseto L2 cache miss or a direct memory access (DMA) data transfer into L2cache 130; (5) data moves from L2 cache 130 to external memory 161 inresponse to a L2 cache victim eviction or writeback and in response to aDMA transfer out of L2 cache 130; (6) data moves from peripherals 169 toL2 cache 130 in response to a DMA transfer into L2 cache 130; and (7)data moves from L2 cache 130 to peripherals 169 is response to a DMAtransfer out of L2 cache 130.

FIG. 2 is a block diagram illustrating details of a digital signalprocessor integrated circuit 200 suitable but not essential for use inthis invention (prior art). The digital signal processor integratedcircuit 200 includes central processing unit 1, which is a 32-biteight-way VLIW pipelined processor. Central processing unit 1 is coupledto level one instruction cache 121 included in digital signal processorintegrated circuit 200. Digital signal processor integrated circuit 200also includes level one data cache 123. Digital signal processorintegrated circuit 200 also includes peripherals 4 to 9. Theseperipherals preferably include an external memory interface (EMIF) 4 anda direct memory access (DMA) controller 5. External memory interface(EMIF) 4 preferably supports access to supports synchronous andasynchronous SRAM and synchronous DRAM. Direct memory access (DMA)controller 5 preferably provides 2-channel auto-boot loading directmemory access. These peripherals include power-down logic 6. Power-downlogic 6 preferably can halt central processing unit activity, peripheralactivity, and phase lock loop (PLL) clock synchronization activity toreduce power consumption. These peripherals also include host ports 7,serial ports 8 and programmable timers 9.

Central processing unit 1 has a 32-bit, byte addressable address space.Internal memory on the same integrated circuit is preferably organizedin a data space including level one data cache 123 and a program spaceincluding level one instruction cache 121. When off-chip memory is used,preferably these two spaces are unified into a single memory space viathe external memory interface (EMIF) 4.

Level one data cache 123 may be internally accessed by centralprocessing unit 1 via two internal ports 3 a and 3 b. Each internal port3 a and 3 b preferably has 32 bits of data and a 32-bit byte addressreach. Level one instruction cache 121 may be internally accessed bycentral processing unit 1 via a single port 2 a. Port 2 a of level oneinstruction cache 121 preferably has an instruction-fetch width of 256bits and a 30-bit word (four bytes) address, equivalent to a 32-bit byteaddress.

Central processing unit 1 includes program fetch unit 10, instructiondispatch unit 11, instruction decode unit 12 and two data paths 20 and30. First data path 20 includes four functional units designated L1 unit22, S1 unit 23, M1 unit 24 and D1 unit 25 and 16 32-bit A registersforming register file 21. Second data path 30 likewise includes fourfunctional units designated L2 unit 32, S2 unit 33, M2 unit 34 and D2unit 35 and 16 32-bit B registers forming register file 31. Thefunctional units of each data path access the corresponding registerfile for their operands. There are two cross paths 27 and 37 permittingaccess to one register in the opposite register file each pipelinestage. Central processing unit 1 includes control registers 13, controllogic 14, and test logic 15, emulation logic 16 and interrupt logic 17.

Program fetch unit 10, instruction dispatch unit 11 and instructiondecode unit 12 recall instructions from level one instruction cache 121and deliver up to eight 32-bit instructions to the functional unitsevery instruction cycle. Processing occurs simultaneously in each of thetwo data paths 20 and 30. As previously described each data path hasfour corresponding functional units (L, S, M and D) and a correspondingregister file containing 16 32-bit registers. Each functional unit iscontrolled by a 32-bit instruction. The data paths are further describedbelow. A control register file 13 provides the means to configure andcontrol various processor operations.

FIG. 3 illustrates the pipeline stages 300 of digital signal processorcore 110 (prior art). These pipeline stages are divided into threegroups: fetch group 310; decode group 320; and execute group 330. Allinstructions in the instruction set flow through the fetch, decode, andexecute stages of the pipeline. Fetch group 310 has four phases for allinstructions, and decode group 320 has two phases for all instructions.Execute group 330 requires a varying number of phases depending on thetype of instruction.

The fetch phases of the fetch group 310 are: Program address generatephase 311 (PG); Program address send phase 312 (PS); Program accessready wait stage 313 (PW); and Program fetch packet receive stage 314(PR). Digital signal processor core 110 uses a fetch packet (FP) ofeight instructions. All eight of the instructions proceed through fetchgroup 310 together. During PG phase 311, the program address isgenerated in program fetch unit 10. During PS phase 312, this programaddress is sent to memory. During PW phase 313, the memory read occurs.Finally during PR phase 314, the fetch packet is received at CPU 1.

The decode phases of decode group 320 are: Instruction dispatch (DP)321; and Instruction decode (DC) 322. During the DP phase 321, the fetchpackets are split into execute packets. Execute packets consist of oneor more instructions which are coded to execute in parallel. During DPphase 322, the instructions in an execute packet are assigned to theappropriate functional units. Also during DC phase 322, the sourceregisters, destination registers and associated paths are decoded forthe execution of the instructions in the respective functional units.

The execute phases of the execute group 330 are: Execute 1 (E1) 331;Execute 2 (E2) 332; Execute 3 (E3) 333; Execute 4 (E4) 334; and Execute5 (E5) 335. Different types of instructions require different numbers ofthese phases to complete. These phases of the pipeline play an importantrole in understanding the device state at CPU cycle boundaries.

During E1 phase 331, the conditions for the instructions are evaluatedand operands are read for all instruction types. For load and storeinstructions, address generation is performed and address modificationsare written to a register file. For branch instructions, branch fetchpacket in PG phase 311 is affected. For all single-cycle instructions,the results are written to a register file. All single-cycleinstructions complete during the E1 phase 331.

During the E2 phase 332, for load instructions, the address is sent tomemory. For store instructions, the address and data are sent to memory.Single-cycle instructions that saturate results set the SAT bit in thecontrol status register (CSR) if saturation occurs. For single cycle 16by 16 multiply instructions, the results are written to a register file.For M unit non-multiply instructions, the results are written to aregister file. All ordinary multiply unit instructions complete duringE2 phase 322.

During E3 phase 333, data memory accesses are performed. Any multiplyinstruction that saturates results sets the SAT bit in the controlstatus register (CSR) if saturation occurs. Store instructions completeduring the E3 phase 333.

During E4 phase 334, for load instructions, data is brought to the CPUboundary. For multiply extension instructions, the results are writtento a register file. Multiply extension instructions complete during theE4 phase 334.

During E5 phase 335, load instructions write data into a register. Loadinstructions complete during the E5 phase 335.

FIG. 4 illustrates an example of the instruction coding of instructionsused by digital signal processor core 110 (prior art). Each instructionconsists of 32 bits and controls the operation of one of the eightfunctional units. The bit fields are defined as follows. The creg field(bits 29 to 31) is the conditional register field. These bits identifywhether the instruction is conditional and identify the predicateregister. The z bit (bit 28) indicates whether the predication is basedupon zero or not zero in the predicate register. If z=1, the test is forequality with zero. If z=0, the test is for nonzero. The case of creg=0and z=0 is treated as always true to allow unconditional instructionexecution. The creg field is encoded in the instruction opcode as shownin Table 1.

TABLE 1 Conditional creg z Register 31 30 29 28 Unconditional 0 0 0 0Reserved 0 0 0 1 B0 0 0 1 z B1 0 1 0 z B2 0 1 1 z A1 1 0 0 z A2 1 0 1 zA0 1 1 0 z Reserved 1 1 1 x

Note that “z” in the z bit column refers to the zero/not zero comparisonselection noted above and “x” is a don't care state. This coding canonly specify a subset of the 32 registers in each register file aspredicate registers. This selection was made to preserve bits in theinstruction coding.

The dst field (bits 23 to 27) specifies one of the 32 registers in thecorresponding register file as the destination of the instructionresults.

The scr2 field (bits 18 to 22) specifies one of the 32 registers in thecorresponding register file as the second source operand.

The scr1/cst field (bits 13 to 17) has several meanings depending on theinstruction opcode field (bits 3 to 12). The first meaning specifies oneof the 32 registers of the corresponding register file as the firstoperand. The second meaning is a 5-bit immediate constant. Depending onthe instruction type, this is treated as an unsigned integer and zeroextended to 32 bits or is treated as a signed integer and sign extendedto 32 bits. Lastly, this field can specify one of the 32 registers inthe opposite register file if the instruction invokes one of theregister file cross paths 27 or 37.

The opcode field (bits 3 to 12) specifies the type of instruction anddesignates appropriate instruction options. A detailed explanation ofthis field is beyond the scope of this invention except for theinstruction options detailed below.

The s bit (bit 1) designates the data path 20 or 30. If s=0, then datapath 20 is selected. This limits the functional unit to L1 unit 22, S1unit 23, M1 unit 24 and D1 unit 25 and the corresponding register file A21. Similarly, s=1 selects data path 20 limiting the functional unit toL2 unit 32, S2 unit 33, M2 unit 34 and D2 unit 35 and the correspondingregister file B 31.

The p bit (bit 0) marks the execute packets. The p-bit determineswhether the instruction executes in parallel with the followinginstruction. The p-bits are scanned from lower to higher address. If p=1for the current instruction, then the next instruction executes inparallel with the current instruction. If p=0 for the currentinstruction, then the next instruction executes in the cycle after thecurrent instruction. All instructions executing in parallel constitutean execute packet. An execute packet can contain up to eightinstructions. Each instruction in an execute packet must use a differentfunctional unit.

FIG. 5 illustrates the details of plural cache lines such as used in L1Icache 121, L1D cache 123 and L2 cache 131 illustrated in FIG. 1. Cache500 illustrated in FIG. 5 includes cache lines 510, 520 and 520 arerepresentative of the internal structure of cache 500. Each of cachelines 510, 520 and 530 includes: respective address tags 511, 521 and522; respective valid bits 512, 522 and 523; respective dirty bits 513,523 and 533; respective least recently used (LRU) indicators 514, 524and 534; and respective data words 515, 525 and 535. Each cache line510, 520 and 530 includes plural respective data words 515, 525 and 535.The bit length of data words 515, 525 and 535 is set by the minimaladdressable data amount of CPU 110. This is typically 8 bits/1 byte.

Cache 500 stores data from more distant memories such as external memory131 which are accessed by a multi-bit address. Cache 500 is organized tofacilitate this storage and to facilitate finding such data in thecache. Each cache line 510, 520 and 530 typically stores 2^(N)respective data words 515, 525 and 535, when N is an integer. Theposition of data words 515, 525 and 535 within the corresponding cacheline 510, 520 and 530 along the dimension 501 serves as a proxy for theleast significant bits of the address.

The position of cached data within lines along dimension 502 serves as aproxy for the next most significant bits of the address. Thecorresponding address tags 511, 521 and 531 form the remainder of thedata word address. To determine if a memory access is to data cachedwithin cache 500 (a cache hit), cache 500 compares the address tags forall cache lines to the most significant bits of the memory locationaccessed. Upon a detecting a match, the position within the cache linealong dimension 501 corresponds to the least significant bits of theaddress permitting identification of the data word accessed.

Each data word 510, 520 and 530 includes a corresponding valid bit 512,522 and 532. A first state of this valid bit indicates the correspondingdata words 515, 525 or 535 are valid. An opposite state of this validbit indicates the corresponding data words 515, 525 or 535 are notvalid. There are several instances where data stored within cache 500would not be valid. Upon initial activation of digital signal processorsystem 100 the L1I cache 121, L1D 123 cache and L2 cache 131 would notbe loaded. Thus they would not store valid data. Accordingly, all cachelines are initially marked invalid. During a cache access a match of arequested address with address tags 511, 521 or 531 would not detect amatch unless the corresponding valid bit 512, 522 or 532 indicated thedata was valid.

Each data word 510, 520 and 530 includes a corresponding dirty bit 513,523 and 533. A first state of this valid bit indicates the correspondingdata words 515, 525 or 535 are dirty. An opposite state of this validbit indicates the corresponding data words 515, 525 or 535 are not dirty(clean). Cache memory is generally used for both read accessed and writeaccesses. Upon a cache hit for a write access, the write data is writteninto the corresponding location within cache 500. According to thepreferred writeback technique, this write data is not immediatelyforwarded to external memory 131. Instead the respective dirty bit 513,523 or 533 is set to indicate dirty. A dirty indication means that therehas been a write to the cached data not currently reflected in the basememory. According to the writeback technique this data is written to thebase memory with the expectation that this writeback can accumulateplural writes to the memory location and nearby memory locations withinthe same cache line to reduce traffic on the bus to external memory 131.

The least recently used (LRU) bits 514, 524 and 534 are used when acache line is replaced. Because the cache cannot hold all the datastored in the large, slow memory, the data within the cache must bereplaced with new data regularly. Using a data words location withindimensions 501 and 502 as proxy for the least significant bitsintroduces a problem in locating data within cache 500. If there is onlya single cache line having the same location on dimensions 501 and 502,then plural data from the large, slow memory will alias to the samecache line in cache 500. This is data having the same least significantaddress bits corresponding to dimensions 501 and 502 but differing mostsignificant address bits. An access to such aliased data would requirethe previous data at that cache line to be replaced. This is considereddisadvantageous. A typical prior art cache is set associative. Thus aset of cache lines have the same location on dimensions 501 and 502.Typical sets include two members (two-way set associative) or fourmembers (four-way set associative). Each cache line of such a set iscalled a way. A cache miss to an address that aliases to one of thesesets needs only to evict one of these ways. Determination of which wayto evict is typically made based on prior usage of these ways. Accordingto both the temporal and spatial locality principles more recently usedcache ways are more likely to be reused than less recently used cacheways. LRU bits 514, 524 and 534 track accesses to cache ways within theset. When data is to be replaced the LRU bits indicate the leastrecently used way for replacement. Maintaining cache coherence requireswriteback of a dirty way upon such replacement.

FIG. 6 is a block diagram illustrating a computing system including alocal memory arbiter according to an embodiment of the invention. FIG. 6illustrates system on a chip (SoC) 600. SoC 600 includes one or more DSPcores 610, SRAM/Caches 620 and shared memory 630. SoC 600 is preferablyformed on a common semiconductor substrate. These elements can also beimplemented in separate substrates, circuit boards and packages. Forexample shared memory 630 could be implemented in a separatesemiconductor substrate. FIG. 6 illustrates four DSP cores 610, but SoC600 may include fewer or more DSP cores 610.

Each DSP core 610 preferably includes a level one data cache such as L1SRAM/cache 612. In the preferred embodiment each L1 SRAM/cache 612 maybe configured with selected amounts of memory directly accessible by thecorresponding DSP core 610 (SRAM) and data cache. Each DSP core 610 hasa corresponding level two combined cache L2 SRAM/cache 620. As with L1SRAM/cache 612, each L2 SRAM/cache 620 is preferably configurable withselected amounts of directly accessible memory (SRAM) and data cache.Each L2 SRAM/cache 620 includes a prefetch unit 622. Each prefetch unit622 prefetchs data for the corresponding L2 SRAM/cache 620 based uponanticipating the needs of the corresponding DSP core 610. Each DSP core610 is further coupled to shared memory 630. Shared memory 630 isusually slower and typically less expensive memory than L2 SRAM/cache620 or L1 SRAM/cache 610. Shared memory 630 typically stores program anddata information shared between the DSP cores 610.

In various embodiments, each DSP core 610 includes a corresponding localmemory arbiter 624 for reordering memory commands in accordance with aset of reordering rules. Each local memory arbiter 624 arbitrates andschedules memory requests from differing streams at a local level beforesending the memory requests to central memory arbiter 634. A localmemory arbiter 624 may arbitrate between more than one DSP core 610.Central memory arbiter 634 controls memory accesses for shared memory630 that are generated by differing DSP cores 610 that do not share acommon local memory arbiter 624.

FIG. 7 is a further view of the digital signal processor system 100 ofthis invention. CPU 110 is bidirectionally connected to L1I cache 121and L1D cache 123. L1I cache 121 and L1D cache 123 are shown togetherbecause they are at the same level in the memory hierarchy. These levelone caches are bidirectionally connected to L2 130. L2 cache 130 is inturn bidirectionally connected to external memory 161 and peripherals169. External memory 161 and peripherals 169 are shown together becausethey are at the same level in the memory hierarchy. Data transfers intoand out of L1D cache 123 is controlled by data memory controller (DMC)710. Data transfers into and out of L1I cache 121 is controlled byprogram memory controller (PMC) 720. Data transfers into and out of L2130 including both cache and directly addressable memory (SRAM) arecontrolled by unified memory controller (UMC) 730.

FIG. 7 illustrates further details of DMC 710 and UMC 730. DMC 710includes L1D cache tags 711. These tags are the non-data part of cachelines 510, 520 and 530 illustrated in FIG. 5 for L1D cache 123. UMC 730includes two sets of cache tags. The first set of cache tags are L2 tags731. These are non-data part of cache lines 510, 520 and 530 illustratedin FIG. 5 for L2 cache 130. The second set of cache tags are shadow tags732. As shown by bus 715 shadow tags 732 generally correspond to L1Dcache tags 711 except these are located in UMC 730. Shadow tags 732include at least the valid and dirty status of the corresponding cachelines in L1D cache 123.

DMC 710 uses shadow tags 732 to implement snoop read and writecoherence. DMC 710 tracks the status of L1D cache lines. Shadow tags 732are used only for snoops intending to keep L2 SRAM coherent with thelevel one data cache. Thus updates for all external cache lines areignored. Shadow tags 732 are updated on all L1D cache allocates and alldirty and invalidate modifications to data stored in L2 SRAM. Theseinteractions happen on different interfaces, but the traffic on thatinterface includes level one data cache accesses to both external andlevel two directly addressable lines. These interactions create extratraffic on these interfaces and creating extra stalls to the CPU. Thusin this invention shadow tags 732 are updated only on a subset of lessthan all updates of the level one tags.

These tag updates are pipelined out from DMC 710 to UMC 730. Commandordering requirements require DMC 710 to ensure that all tag updateprior to a particular command have been send out to shadow tags 732. L1Dcache 123 and CPU 110 stall until that is complete. The performanceimprovement from this invention is a result of having to do this veryfew times. Additional improvements come from reduced dynamic powerconsumption as a result of largely reduced accesses to the shadow tags732.

FIGS. 8A and 8B together form a flow chart illustrating when shadow tags732 in UMC 730 updates. In accordance with this invention shadow tags732 are not updated every time the corresponding L1D cache tags 711update. FIG. 8 illustrates a test loop including test blocks 801, 803,807, 811 and 815. Test block 801 determines if L1D cache 123 allocatesto L2 SRAM. If this occurs (Yes at test block 801), then the shadow tag732 for the corresponding line is marked valid and clean (block 802).Flow returns to the top of test loop.

Test block 803 determines if L1D cache 123 allocates a line to externalmemory. If this occurs (Yes at test block 803), then test block 804determines if the allocated address is cacheable. If this does notoccurs (No at test block 803), then the corresponding line in shadowtags 732 is unchanged (block 805). Flow returns to the top of test loop.If this occurs (Yes at test block 804), then the shadow tag 732 for thecorresponding line is invalidated (block 806). Flow returns to the topof test loop.

Test block 807 determines if the status of a L1D cache line moves fromclean to dirty. If this occurs (Yes at test block 807), then test block808 determines if the line was stored in L2 SRAM. If this does not occur(No at test block 808), then the corresponding line in shadow tags 732is unchanged (block 809). Flow returns to the top of test loop. If thisoccurs (Yes at test block 808), then the shadow tag 732 for thecorresponding line changes to dirty (block 810). Flow returns to the topof test loop.

Test block 811 determines if a L1D cache line is invalidated. If thisoccurs (Yes at test block 811), then test block 812 determines if theline was stored in L2 SRAM. If this does not occur (No at test block812), then the corresponding line in shadow tags 732 is unchanged (block813). Flow returns to the top of test loop. If this occurs (Yes at testblock 812), then the shadow tag 732 for the corresponding line changesto invalid (block 814). Flow returns to the top of test loop.

Test block 815 determines if a L1D cache line is evicted. If this occurs(Yes at test block 815), then test block 816 determines if the evictionis accompanied by an allocate and if the new line is stored in L2 SRAM.If this does not occur (No at test block 816), then the correspondingline in shadow tags 732 is changes to clean and the tag is replaced withthe new value (block 817). Flow returns to the top of test loop. If thisoccurs (Yes at test block 816), then test block 818 determines if theexternal address is cacheable. If this does not occur (No at test block818), then the corresponding line in shadow tags 732 is unchanged (block819). If this occurs (Yes at test block 818), then the correspondingline in shadow tags 732 is changes to invalid (block 820). Flow returnsto the top of test loop.

In the prior art all L1D allocates, status changes to dirty or invalidwere updated in the L2 tag shadow copy. This interaction did not changebased on which line was being allocated (L2 SRAM, external) and itscacheability.

A large percentage of L1D line allocates are for external addresses.This invention causes a large drop in the number of updates to the L2tag RAM shadow copy in the L2 controller. This invention increasesperformance and decreases CPU stalls. This invention is advantageous byavoiding unnecessary tag shadow tag updates. This creates a largeimprovement in performance.

1. A data processing system comprising: a central processing unitexecuting program instructions to manipulate data; a first level datacache connected to said central processing unit temporarily storing in aplurality of cache lines data for manipulation by said centralprocessing unit, each cache line including a tag indicating a valid anda dirty status of said data stored therein; a first level cacheoperation unit connected to said first level cache capable controllingdata transfers into and out of said first level cache including controlof said tags; a second level cache connected to said first level cachetemporarily storing in a plurality of cache lines data for manipulationby said central processing unit; and a second level cache operation unitconnected to said first level cache operation unit and said second levelcache controlling data transfers into and out of said second levelcache, said second level cache including a set of shadow tagscorresponding to said tags of said first level data cache, said secondlevel cache operation unit updating said shadow tags on a subset of lessthan all changes to corresponding tags of said first data cache.
 2. Thedata processing system of claim 1, wherein: said second level cachefurther includes memory directly addressable by said central processingunit; and said second level cache operation unit sets a shadow tag tovalid and clean if said first level cache operation controller allocatesa tag of a corresponding first level cache line corresponding to anaddress within said memory directly addressable by said centralprocessing unit.
 3. The data processing system of claim 1, wherein: saidsecond level cache operation unit invalidates a shadow tag if said firstlevel cache operation allocates a tag of a corresponding first levelcache line to an external address if said corresponding external addressis cacheable.
 4. The data processing system of claim 1, wherein: saidsecond level cache further includes memory directly addressable by saidcentral processing unit; and said second level cache operation unit setsa shadow tag to dirty if said first level cache operation controllerchanges a tag of a corresponding first level cache line corresponding toan address within said memory directly addressable by said centralprocessing unit from clean to dirty.
 5. The data processing system ofclaim 1, wherein: said second level cache further includes memorydirectly addressable by said central processing unit; and said secondlevel cache operation unit invalidates a shadow tag if said first levelcache operation controller invalidates a tag of a corresponding firstlevel cache line corresponding to an address within said memory directlyaddressable by said central processing unit.
 6. The data processingsystem of claim 1, wherein: said second level cache further includesmemory directly addressable by said central processing unit; and saidsecond level cache operation unit changes a shadow tag to indicate cleanif said first level cache operation controller evicts a first levelcache line, which is further not allocated to said memory directlyaddressable by said central processing unit.
 7. The data processingsystem of claim 1, wherein: said second level cache further includesmemory directly addressable by said central processing unit; and saidsecond level cache operation unit invalidates a shadow tag if said firstlevel cache operation controller evicts a tag of a corresponding firstlevel cache line corresponding to an address within said memory directlyaddressable by said central processing unit, which is further allocatedto said memory directly addressable by said central processing unit atan external cacheable address.
 8. The data processing system of claim 1,wherein: said second level cache further includes memory directlyaddressable by said central processing unit; and said second level cacheoperation unit updates a shadow tag if said first level cache operationcontroller updates a tag of a corresponding first level cache linecorresponding to an address within said memory directly addressable bysaid central processing unit.
 9. The data processing system of claim 8,wherein: said first level cache operation controller updates a tag of afirst level cache line to an address within said memory directlyaddressable by said central processing unit including allocating a cacheline, changing a cache line from clean to dirty, invalidating a cacheline and evicting a cache line.